1. Field of the Invention
The present invention relates to an antenna apparatus for a small and thin wireless device, and more particularly, to a technique for implementing an antenna on a high-impedance substrate.
2. Related Art
An Electromagnetic Band Gap (EBG) substrate is known as a technique for arranging a metallic plate (a ground plate) and an antenna in proximity to each other for the purpose of making an antenna apparatus thin. An EBG substrate is structured by arranging conductor plates in a matrix at a certain height on a metallic plate and each of the conductor plates is connected with the metallic plate by a linear conductive element. The EBG substrate realizes high impedance by creating LC parallel resonance circuits by way of distributed constant circuits so as to suppress unnecessary current distribution generated on the metallic plate.
However, since a current distributes also on the EBG substrate, degradation of antenna characteristics occurs when the EBG substrate and the antenna are arranged very closely to each other. This is because current distribution on the antenna significantly varies due to the effect of current distributing on the EBG substrate, which makes matching impossible. A steep change of current in the vicinity of a feeding point in particular causes a significant degradation of matching characteristics.
Therefore, EBG substrates generally suppress characteristic variation resulting from mutual coupling by not positioning the antenna and the EBG substrate very closely to each other. Such a method has a limit on reduction of the thickness of an antenna apparatus.
JP-A 2005-110273 (Kokai) describes a method which removes one unit cell of an EBG substrate and places an antenna therein. However, such a placement as described in the publication becomes a cause of hindering the reduction of antenna thickness, which is a goal primarily pursued by the EBG substrate. Also, when the size of unit cells of the EBG substrate is relatively large, an unnecessary current induced by a current on the antenna is generated on the EBG substrate.
U.S. Pat. No. 6,768,476 discloses a method for arranging antennas in gaps between conductor plates, which are considered to be little affected by current on the EBG substrate. However, this technique also has a problem that current distribution changes due to influence of current on the EBG substrate and impedance matching characteristic of antennas significantly degrades.